Apparatus for treating a liquid with a gas can have many utilities and of particularly great interest at the present time are systems for dissolving oxygen in liquids such as waste water or sewage as part of a purification thereof.
Waste-water purification systems may involve the aerobic decomposition of organic components of waste water, e.g. in an activated sludge tank, with the result that a purified decantate can be removed free from toxic or noxious components. Of course, it is also interesting to introduce other gases, such as carbon dioxide, into water for various purposes, to dissolve ammonia in water and, in general to treat a liquid phase with gas phase.
A wide variety of techniques have been developed for this purpose, ranging from the simple injection of the gas into a stationary body of water through so-called aeration stones, spraying the liquid into the gas, rapidly mixing liquid in such a manner as to draw gas into it, etc.
A particularly advantageous approach has been described in the German published application (Offenlegungsschrift) DT-OS No. 2 223 460 in which the liquid is pumped through a multiplicity of passages into which capillary ribs open to introduce the gas. The result is a highly efficient gas diffusion system in which the liquid streams passing through the channels entrain the gas in the form of small bubbles. The bubble size depends upon the velocity of the liquid stream flowing across the lateral opening of the capillary into the flow passage because the bubble is formed by a shearing at this opening.
When such a system is used for waste waters and other contaminant-containing liquid streams, the flow passages are readily blocked or obstructed and the necessary high velocities cannot be obtained.
Thus, while the system is highly efficient if satisfactory liquid velocities and flow rates can simultaneously be attained, it has not been found to be practical for many applications.